PCIe (Peripheral Component Interconnect Express) is a high-speed serial computer expansion bus standard. PCI Express operates in consumer, server, and industrial applications. The PCIe bus is like a high-speed serial replacement of the older PCI/PCI-X bus, an interconnect bus using shared address/data lines. PCI uses a shared parallel bus architecture, where the PCI host and all devices share a common set of address/data/control lines. In terms of bus protocol, PCIe communication is encapsulated in packets. The work of packetizing and de-packetizing data and status-message traffic is handled by the transaction layer of the PCIe port.
NVMe (Non-Volatile Memory Express) is a specification for accessing solid-state drives (SSDs) on a PCIe bus. NVM Express is an optimized, high performance, scalable host controller interface with a streamlined register interface and command set designed for enterprise and client systems that use PCI Express SSDs.
AHCI (Advanced Host Controller Interface) is a technical standard that specifies the operation of Serial ATA (SATA) host bus adapters in a non-implementation-specific manner. AHCI benefits from legacy software compatibility, it does not deliver optimal performance when talking to a PCI Express SSD. AHCI was developed at a time when the purpose of the Host Bus Adapter (HBA) in a system was to connect the CPU/memory subsystem with the much slower storage subsystem based on rotating magnetic media. Such an interface has some inherent inefficiency when applied to SSD devices, which behave much more like DRAM than spinning media. At a high level, the basic advantages of NVMe over AHCI relate to the ability to exploit parallelism in host hardware and software.
SOP (SCSI (Small Computer System Interface) over PCIe) attempts to standardize the SCSI protocol across a PCIe physical interface. SOP supports PQI (PCIe Queuing Interface). SOP allows for re-use of existing storage industry testing technologies for testing PCIe SSDs.